1. Field of the Invention
This invention relates to an image evaluating method and apparatus, wherein an image, which has been obtained from an image recording apparatus, an image processing unit, or the like, is evaluated. This invention also relates to a computer program for causing a computer to execute the image evaluating method, and a computer readable recording medium, on which the computer program has been recorded.
2. Description of the Related Art
It is one of important subjects for image processing algorithm developers how to evaluate effects of image processing during activities for developing new image processing algorithms for image processing, such as enhancement processing in a frequency domain, smoothing processing, or noise removal processing.
As an index for representing image quality, a detective quantum efficiency value (DQE value) has heretofore been utilized. The DQE value is the value representing the image quality and is calculated with the formula shown below. A large DQE value indicates enhanced image quality.DQE(u)={(log10e)2×γ2×MTF2(u)}/{q×WS(u)}wherein MTF represents the modulation transfer function, which is obtained by recording the image of the contrast transfer function chart (CTF chart) and is the index for representing the level of resolution, i.e. the sharpness, of the image signal of each of frequency bands, WS(u) represents the Wiener spectrum and is the index for representing the graininess, u represents the frequency, γ represents the gamma value representing the gradation characteristics of the film utilized for obtaining the image, and q represents the quantum number.
In the cases of linear image processing, the sharpness and the graininess are improved in the same manner, and therefore the DQE value itself does not vary. In the cases of nonlinear image processing, the sharpness and the graininess are capable of being processed as the two independent processes, and therefore the DQE value is capable of being improved markedly, depending upon whether the image processing with respect to the sharpness or the image processing with respect to the graininess is or is not performed.
As one of typical techniques for evaluating image quality of images in the fields of radiation images, a visual evaluating technique utilizing a Burger phantom has heretofore been known. The Burger phantom comprises a plurality of arrayed circular cylinder-shaped acrylic bodies, whose diameters and thicknesses vary by stages. In cases where an image obtained from an imaging operation performed on the Burger phantom by use of an imaging system is seen, both the spatial resolution and the contrast resolution of the image obtained with the imaging system are capable of being evaluated. The diameter of each of the circular cylinder-shaped acrylic bodies acts as the index for the evaluation of the spatial resolution. Also, the thickness of each of the circular cylinder-shaped acrylic bodies acts as the index for the evaluation of the contrast resolution. In cases where the image of the Burger phantom is obtained with an imaging system, which is capable of yielding an image having good image quality, an image pattern of a circular cylinder-shaped acrylic body, which has a small diameter and a small thickness, is capable of being perceived in the image of the Burger phantom. Therefore, the image of the Burger phantom may be obtained with an imaging system, for which the image evaluation is to be performed, and the limit of perceptibility of an image pattern, which corresponds to a circular cylinder-shaped acrylic body, in the image of the Burger phantom may be recognized. In this manner, the evaluation of the image obtained with the imaging system is capable of being performed.
However, the Burger phantom comprises the circular cylinder-shaped acrylic bodies, which are located at predetermined positions. Therefore, the person, who sees the image of the Burger phantom, recognizes previously that the image patterns of the circular cylinder-shaped acrylic bodies will be present at the positions in the image, which positions correspond to the predetermined positions of the circular cylinder-shaped acrylic bodies in the Burger phantom. Accordingly, psychological effects occur with the person, who sees the image of the Burger phantom, in that an image pattern of a circular cylinder-shaped acrylic body, which image pattern is actually invisible, seems to be visible. The problems thus occur in that the results of the image evaluation are adversely affected by the psychological effects occurring with the person, who sees the image of the Burger phantom.
Therefore, in lieu of the Burger phantom, a CDRAD phantom (i.e., a contrast detail digital/conventional radiography phantom, supplied by Northwest X-ray Inc.) is recently utilized for the image evaluation.
The CDRAD phantom is constituted of an acrylic plate, which is divided into 15×15 regions arrayed in a lattice-like pattern. One or two holes are formed within each of the lattice regions. The diameters and the depths of the holes vary by stages for different lattice regions. Specifically, the holes within the lattice regions are formed such that the diameters of the holes vary by stages in the vertical direction of the array of the lattice regions, and such that the depths of the holes vary by stages in the horizontal direction of the array of the lattice regions. Also, as for the lattice regions, which are located along the top row, the second top row, and third top row in the array of the lattice regions, one hole is formed within each of the lattice regions. As for the lattice regions, which are located along the other rows in the array of the lattice regions, two holes are formed within each of the lattice regions. More specifically, in each of the lattice regions, which are located along the other rows in the array of the lattice regions, a first hole having a certain diameter and a certain depth is formed at a center area of the lattice region, and a second hole having a diameter identical with the diameter of the first hole and a depth identical with the depth of the first hole is formed at one of four corner areas of the lattice region. When the image evaluation with the CDRAD phantom is to be performed with respect to an imaging system, a radiation image of the CDRAD phantom is acquired with the imaging system, and a CDRAD phantom image is thereby obtained. Evaluation of the CDRAD phantom image is then performed. In this manner, evaluation of images obtained with the imaging system, with which the radiation image of the CDRAD phantom has been acquired, is capable of being performed. Further, in cases where image processing is performed on the CDRAD phantom image by use of an image processing system, and a processed CDRAD phantom image is thereby obtained, evaluation of images obtained with the image processing system is capable of being performed. How the image evaluation with the CDRAD phantom is performed will be described hereinbelow.
FIG. 2 is an explanatory view showing an example of a radiation image of a CDRAD phantom (hereinbelow referred to as the CDRAD phantom image). The holes within the lattice regions constituting the CDRAD phantom are formed such that the diameter of each of the holes becomes small toward the bottom row in the array of the lattice regions, and such that the depth of each of the holes becomes small toward the left-hand end column in the array of the lattice regions. Therefore, as illustrated in FIG. 2, as the location of the lattice region becomes close to the lower left-hand corner of the CDRAD phantom, the image pattern of the hole formed within the lattice region becomes hard to see. In the CDRAD phantom image illustrated in FIG. 2, the numerals indicated on the left-hand side of the lattice regions represent the diameters of the holes. Also, the numerals indicated on the bottom side of the lattice regions represent the depths of the holes.
Firstly, with respect to each of the top row, the second top row, and third top row in the array of the lattice regions, the person, who sees the CDRAD phantom image, indicates the limit of visibility of the hole image patterns. Also, with respect to each of the lattice regions, which are located along the other 12 lower rows in the array of the lattice regions, the person, who sees the CDRAD phantom image, indicates the position of the image pattern of the hole, which is formed at one of the four corner areas of the lattice region. Specifically, the person, who sees the CDRAD phantom image, indicates that the image pattern of the hole is located at the upper right-hand corner area, at the lower right-hand corner area, at the upper left-hand corner area, or at the lower left-hand corner area.
After the indication has been made by the person, who sees the CDRAD phantom image, comparisons between the results of the indication and the true positions are made with respect to all lattice regions (15×15=225 lattice regions). In this manner, trueness-falseness results are obtained with respect to each of the lattice regions.
It may occur with a probability of ¼ that the result of the indication coincides by accident with the true position. In order for the problems to be eliminated, the trueness-falseness results are corrected in accordance with the rules described under (1) to (4) below.    (1) In cases where the result of the indication having been made with respect to a lattice region (hereinbelow referred to as the lattice region of interest), for which the judgment as to the trueness or falseness of the result of the indication is to be made, has been found to coincide with the true position, if the results of the indications having been made with respect to at least two lattice regions, which are among the four (i.e., upper, lower, right-hand, and left-hand) nearest neighbor lattice regions, are found to coincide with the true positions, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position. In cases where a lattice region, which is in contact with a side of the CDRAD phantom image, is taken as the lattice region of interest, if the results of the indications having been made with respect to at least two lattice regions, which are among the three nearest neighbor lattice regions, are found to coincide with the true positions, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position.    (2) In cases where only two nearest neighbor lattice regions exist, i.e. in cases where a lattice region located at one of the four corner areas of the CDRAD phantom image is taken as the lattice region of interest, and the result of the indication having been made with respect to the lattice region of interest has been found to coincide with the true position, if the result of the indication having been made with respect to one of the two nearest neighbor lattice regions is found to coincide with the true position, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position.    (3) In cases where the result of the indication having been made with respect to the lattice region of interest has been found not to coincide with the true position, if the results of the indications having been made with respect to at least three lattice regions, which are among the four nearest neighbor lattice regions, are found to coincide with the true positions, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position. In cases where a lattice region, which is in contact with a side of the CDRAD phantom image, is taken as the lattice region of interest, and the result of the indication having been made with respect to the lattice region of interest has been found not to coincide with the true position, if the results of the indications having been made with respect to all of the three nearest neighbor lattice regions are found to coincide with the true positions, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position.    (4) In cases where a lattice region located at one of the four corner areas of the CDRAD phantom image is taken as the lattice region of interest, and the result of the indication having been made with respect to the lattice region of interest has been found not to coincide with the true position, if the results of the indications having been made with respect to both the two nearest neighbor lattice regions are found to coincide with the true positions, it should be regarded that the result of the indication having been made with respect to the lattice region of interest coincides with the true position.
Also, reference is made to the corrected trueness-falseness results, and the limit values of the diameters and the depths of the visually perceptible holes are plotted on a logarithmic graph, in which the logarithmic values of the hole depths are plotted on the horizontal axis, and the logarithmic values of the hole diameters are plotted on the vertical axis. In this manner, a CD curve is obtained, where C represents the contrast, and D represents the detail. With the CD curve, it is capable of being found that, in cases where an image pattern of a hole, which has a small diameter and a small depth, is visually perceptible in the CDRAD phantom image, the imaging system which yielded the CDRAD phantom image may be regarded as having good performance.
As illustrated in FIG. 2, in the cases of the image evaluation utilizing the CDRAD phantom, the position of the hole, which is located at one of the four corner areas of each of the lattice regions, is not known previously. Therefore, the results of the image evaluation are not apt to be affected by subjectivity of the person, who sees the CDRAD phantom image. As a result, more objective evaluation results are capable of being obtained than with the image evaluation utilizing the Burger phantom.
As described above, the technique for performing the image evaluation by use of the CDRAD phantom requires the procedure for making a judgment as to the trueness or falseness of the result of the indication having been made by the person, who sees the CDRAD phantom image, with respect to the 225 lattice regions per CDRAD phantom image, and thereby obtaining the trueness-falseness results, the procedure for correcting the thus obtained trueness-falseness results, and the procedure for forming the CD curve from the corrected trueness-falseness results. Therefore, the evaluation time per CDRAD phantom image becomes as long as approximately 15 minutes. Also, in the cases of the image evaluation utilizing the CDRAD phantom, as in the cases of the image evaluation utilizing the Burger phantom, it may often occur that the results of the image evaluation are affected by the subjectivity of the person, who sees the CDRAD phantom image. Therefore, such that a statistical difference among the persons, who see the CDRAD phantom image, may be suppressed, it is necessary for the image evaluation of a CDRAD phantom image to be made by at least three persons, who see the CDRAD phantom image. Further, such that a difference in image recording conditions may be suppressed statistically, it is necessary for at least three CDRAD phantom images to be recorded and subjected to the image evaluation. Furthermore, in cases where a value of a parameter for image processing, which is performed on an image signal with an image processing unit, is altered, and the images obtained from the image processing are subjected to the image evaluation, it is necessary for at least three CDRAD phantom images to be prepared with respect to each of the altered values of the parameter. Accordingly, the technique for performing the image evaluation by use of the CDRAD phantom has the problems in that considerable time and labor are required to perform the image evaluation, and the cost for evaluation, such personnel expenses, cannot be kept low.
Also, the image evaluation utilizing the CDRAD phantom is the visual evaluation made by the human. Therefore, the problems occur in that the results of the image evaluation often vary, depending upon the environmental conditions under which the CDRAD phantom image is seen, i.e., the brightness at the site of the image evaluation, the time at which the image evaluation is made, the order in which the CDRAD phantom images are evaluated, the physical condition and the mental condition of the person, who sees the CDRAD phantom image, and the like, and reliable evaluation results cannot be obtained.